This is a false alarm. It is the hashed MAC address of a loopback adapter. Note that on the Mantis, they have different hashed disk ID's.

All Loopback adapters on Linux and PC's use the same Mac address. I found this out when I banned myself.

Please read the note from Tampa, the developer who wrote the code about mac banning in OpenSim in the mantis that i opened. Be careful banning people, this "mac" starting with 288... is related to a very common network adapter present in many devices.

So it seems that the problem of duplicate MACs happen only to people using Firestorm but not when using SIngularity. It must be a Firestorm issue then and not an OpenSim issue.

Good morning again. I have just opened a mantis:
http://opensimulator.org/mantis/view.php?id=9044

Good Morning. I am the owner of Craft World. Please note that those so called 'mac' are not mac and are not unique. You can't identify an user with that mac because a same mac can be shared between many different users. Just browse your Robust.log. In mine I found people from Hungary and from Sardinia sharing the same 'mac' and so on. This horrible issue is generating hysteria and I am going to open a mantis today.

I would have 15 accounts, and in what grids then? and it's still banned? I don't even care about your grill, I only came to R.Lion to decorate my house with avitron and craft-world

yeah that one there is trouble